This invention relates to hard surfacing alloys and, particularly, to iron-base alloys which are cobalt-free and which utilize relatively high levels of nickel and low levels of chromium and boron.
Hard facing is essentially a welding process which forms an alloy surface on a base metal to provide a wear-, heat-, and corrosion-resistant surface for the base metal. Hard facing is used where it is difficult to lubricate the base metal against abrasion or to provide a wear-resistant surface where it is impractical or impossible to harden the surface by conventional heat treating or heat hardening metallurgical techniques. Alloys employed as hard facing materials range from low-cost ferrous material to relatively expensive nickel-, cobalt-, or tungsten-base alloys.
Generally, the hard facing alloys may be classified as low alloy ferrous materials, high alloy ferrous materials, nickel-base alloys, cobalt-base alloys, and tungsten carbide. The low alloy ferrous materials include chromium as the chief alloying constituent in amounts ranging from 1% to 7%, with relatively small amounts of manganese, molybdenum, silicon, and carbon. The high alloy ferrous materials include chromium in amounts up to 15%, with significant amounts of molbydenum and/or manganese. The nickel-base alloys include between 70% and 80% nickel and between 10% and 17% chromium. The cobalt-base alloys have between 45% and 65% cobalt, between 20% and 30% chrome, and between 5% and 50% tungsten.
A widely used hard facing alloy is a high alloy ferrous material utilizing high levels of chromium and carbon (10%-30% and 2%-4% by weight, respectively). These alloys generally depend upon phase transformation and/or formation of acicular chromium carbides for their resistance to wear. At the higher levels of chromium, some increased resistance to corrosive attack can also be noted, despite the elevated levels of carbon normally associated with these materials. These alloys are intended as substitutes for cobalt-base alloys, where high resistance to galling and/or crack-free overlays is required to meet specific service requirements.
While the high chromium, high carbon iron-base alloys form hard, wear-resistant overlays, they have a tendency to crack when applied by the various welding processes. To overcome this problem, additions of cobalt and/or nickel have been made, together with a lowering of the levels of carbon usually employed in these alloys. These efforts have resulted in alloys with satisfactory weldability and lower susceptibility to cracking while retaining some of the performance characteristics of the more expensive cobalt-base materials. These modifications, however, have not significantly increased the resistance to galling of these alloys.
In addition, since the sources for cobalt are quite limited, and this element is extensively used in the aerospace industry, cobalt is considered to be a strategic material. This condition, when coupled with the price pressure of cobalt, has led to efforts to develop alloys which retain the performance characteristics of the cobalt-base alloys while using little or none of this element. An example of such an effort is set forth in U.S. Pat. No. 4,259,111, which relates to a nickel-base alloy containing chromium, silicon, and molybdenum.